1. Field of the Invention
The present invention relates to a position detection apparatus, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
A demand has arisen for an exposure apparatus with higher precision and higher functionality. For example, a technique for matching the pattern of a reticle (mask) with a wafer (a pattern formed on it) on the order of nanometers has been demanded in alignment between a reticle and a wafer.
One method of the alignment between a reticle and a wafer is wafer alignment. In the wafer alignment, first, a wafer is loaded into a system including an exposure apparatus and mechanical alignment apparatus. The mechanical alignment apparatus then coarsely aligns the wafer using an orientation flat or notch formed on the periphery of the wafer. The coarsely aligned wafer is placed on a wafer stage of the exposure apparatus by a wafer loader. Note that the mechanical alignment apparatus generally has an alignment accuracy of about 20 μm.
Next, global alignment in which the positions of a plurality of alignment marks formed on the wafer are calculated is performed (see Japanese Patent Laid-Open No. 09-218714). The global alignment serves to align the wafer in accordance with the same correction equation over its entire region. Hence, the alignment state can be examined by observing (detecting) some of the plurality of alignment marks. Note that the global alignment accuracy is 10 nm or less.
In the global alignment, the alignment marks are observed using, for example, two sensors: a low-magnification sensor serving as an area sensor, and a high-magnification sensor serving as a line sensor. The final positions of the alignment marks are determined based on their images sensed by the high-magnification sensor. However, the high-magnification sensor has so narrow an observation field of view that the alignment marks often fall outside the observation field of view. To combat this situation, two low-magnification observation marks formed at left and right positions, respectively, on the wafer are observed using the low-magnification sensor to calculate and correct shifts in the X- and Y-axis directions, rotational components, and shot magnification components of the wafer so that the alignment marks fall within the observation field of view of the high-magnification sensor. Note that alignment using a low-magnification sensor is called search alignment, and that using a high-magnification sensor is called fine alignment.
In the fine alignment, focus adjustment needs to be performed so that the alignment marks each are positioned at a best focus position of the high-magnification sensor in order to detect the alignment marks with high accuracy. In the focusing, images of the alignment marks are sensed while driving (scanning) the wafer stage in the Z-axis direction, and the contrasts of the images of the alignment marks sensed at each Z position are calculated. A Z position where the contrast maximizes is determined as a best focus position.
In exposing each shot on the wafer, the wafer stage is scanned based on the global alignment result, and the pattern formed within the shot is precisely aligned with the pattern of the reticle projected onto the wafer via a projection optical system.
In this manner, in the wafer alignment, the alignment marks are observed by switching a plurality of sensors such as a low-magnification sensor and a high-magnification sensor. Also, a method of observing the alignment marks using one high-resolution sensor in place of a plurality of sensors has recently been proposed. A high-resolution sensor can not only obtain a field of view corresponding to that of a low-magnification sensor using a wide field of view, but also obtain a resolution corresponding to that of a high-magnification sensor. This makes it possible to observe the low-magnification observation marks and the alignment marks using only one sensor.
Unfortunately, since a high-resolution sensor includes a large number of pixels, it takes a long time for this sensor to transfer signals (that is, it takes much time for this sensor to read signals from the large number of pixels, respectively). As a result, it takes much time to sense images of the alignment marks at each Z position in the focusing. This makes it impossible to perform wafer alignment (fine alignment) in a short period of time.